I’m experimenting with CELT compression and file sizes. I could be doing something incredibly stupid here (or missed some documentation somewhere), but it seems that after CELT-compressing the same piece of music at different sample rates, the output bank size seems to be the same.

Does FMOD Designer up-sample everything to 48KHz (or some other arbitrary sample rate) before using CELT compression?

Any bit rate based format does nothing to the file size if the sample rate changes. If you set 128kbps as the bitrate, thats how many bits are going to be written per second.
The main benefit is if you lower the sample rate in your sound, the bitrate can be lowered as well without perceptible loss

Yes, FMOD resamples the sourcedata to 44100 for celt. To adjust the file size you should use the ‘quality’ parameter of the celt encoder. It works the same as MP3 encder: bitrate = (quality * 3.2kbit/s) so a quality of 40 will give you 128 kbit/s.